The proper distribution of liquid fuel in a flowing gas stream is the objective of a wide variety of fuel combusting processes and equipment. One particularly demanding application is in a thrust augmentor for establishing afterburning in the exhaust gas stream of a gas turbine engine.
As the state of the gas turbine art has advanced, the temperature of the exhaust gases exiting the engine have climbed. Higher exhaust gas temperatures have required designers to seek ways to avoid vaporization of the liquid fuel within the augmentor fuel distribution structure disposed in the high temperature exhaust environment. As will be appreciated by those skilled in the art, the two phase flow created by the partial vaporization of a liquid can radically distort the mass flow pattern within a closed conduit or other liquid distribution system.
One prior art solution has been to surround the fuel distributor with an air cooled shield supplied by relatively cool air diverted from the upstream portion of the gas turbine engine. As with the exhaust gas stream, the temperature of this diverted air increases with increasing engine performance, reaching, in certain applications, temperatures at which the "relatively cool" air is too warm to prevent vaporization of the augmentor fuel. Another strategy has been to specify less volatile augmentor fuel for use in such high temperature engines and applications, thus allowing the liquid fuel temperature to be raised an additional amount before the onset of vaporization.
The fuel vaporization problem is especially critical at the initiation of augmentor operation when the augmentor is supplied with a relatively low fuel flow rate in order to avoid a high pressure transient upon ignition. Prior art fuel distributors have been subject to such pressure spikes and other instabilities in the augmentor light off due to flow irregularities caused by the vaporization of the fuel as it first enters the heated distribution structure. Irregular flow continues for that time period during which the augmentor fuel flow is slowly increased to a minimum continuous flow at which the fuel is discharged into the gas stream before absorbing enough heat to vaporize.
As will be appreciated by those skilled in the art, the occurrence of a vapor bubble or two phase mixture in a prior art spraybar may cause the bar to unevenly discharge fuel into the gas stream, possibly causing a wide variation in local fuel to air ratios. Typical augmentor arrangements use one or more igniters disposed in a pilot region near the gas flow centerline within the augmentor for encouraging smooth, reduced pressure transient light off and continued stable augmentor combustion. What is needed is a spraybar which can ensure a proper flow of fuel to this igniter equipped pilot region at initial and relatively low fuel mass flow rates.